1. Field of the Invention
This invention relates to a method and apparatus for pigging, or cleaning, the interior surfaces of flowlines, and more particularly, to the pigging of dissimilar size dual flowlines associated with subsea wells.
2. Description of the Prior Art
The need exists in the offshore production of oil and gas from a subsea satellite well to provide a method and apparatus to pig, or clean, flowlines which typically pass oil and gas products therethrough. It is common in the hydrocarbon production art to remote such deposits and/or debris from flowlines by pumping a tool, or "pig", through the flowlines. The pig scrapes of dislodges the deposits and/or debris from the flowlines so that the deposits and/or debris are carried out of the lines by the fluids flowing therethrough.
Subsea satellite wells may produce heavy crude oil having a pour point of 30.degree. F. or higher. The crude produced may also contain carbon dioxide, hydrogen sulfide, and water. Because of the high pour point and the corrosive components, the production flowline should be periodically pigged for corrosion control, and to remote paraffin deposits. The presence of the hydrogen sulfide, carbon dioxide, and water in the produced crude oil will form a corrosive condition within the flowlines. Pigging is normally used as part of an overall corrosion control program to remove scale from the flowlines and sweep out water that may collect in low spots in the flowlines. Additionally, for wells producing heavy crude oils with relatively high pour points, heavy oil in the flowlines will become very viscous after a well is shut-in because of the high pour point of the crude and the low sea floor temperature (e.g., 45.degree. F.). Thus, pigging may be required prior to production start-up to remove cold, heavy oil, as well as paraffin deposits or scale.
Subsea satellite wells may utilize dissimilar size flowlines to provide high flow rates through the larger flowline, and still allow dual tubing strings to fit within standard oil well casing sizes. Accordingly, the capability to pig subsea satellite well flowlines of dissimilar sizes is of increasing importance, as more subsea well systems are used in non-ideal operating conditions.
At present, it is not believed that there has been proposed, manufactured, or installed, a dedicated subsea satellite well pigging system for dissimilar size dual flowlines. Further, commonly used pigging procedures for subsea satellite wells with dissimilar size flowlines which utilize either standard pigs, such as foam or spherical pigs, or through-the-flowline (TFL) tools rely on fluid circulation within the flowline to remove debris from the flowline. These procedures do not allow the pig to physically push the debris to the surface facility or production platform.
Accordingly, prior to the development of the present invention, there has been no method or apparatus for pigging dissimilar size dual flowlines for subsea satellite wells which does not solely rely upon fluid circulation to remove debris from the flowlines, and allows the pig to physically push various types of deposits and/or debris from the flowline to the surface facility. Therefore, the art has sought a method and apparatus for pigging dissimilar size dual flowlines utilized for subsea satellite wells, which: approximate full loop pigging; do not solely rely upon fluid circulation to remove debris from the flowlines; and allow the pig to physically push deposits and/or debris from within the flowlines to the surface facility.